A desirable solar cell geometry referred to as an interdigitated back contact (IBC) cell comprises a semiconductor wafer and alternating lines (interdigitated stripes) of regions with p-type and n-type doping. This cell geometry has the advantage of eliminating shading losses altogether by putting both contacts on one side of the wafer. Further, contacts are easier to interconnect with both contacts on the rear. A laser-transfer process developed by Mei et al. at Xerox Palo Alto Research Center (PARC) is discussed in U.S. Pat. No. 5,871,826 entitled “Proximity Laser Doping Technique for Electronic Materials” was issued on Feb. 16, 1999. The patent describes a method of altering the electrical characteristics of a material through a laser ablation process, which can achieve high doping levels and shallow junctions at low temperatures. The invention utilizes a rapid interaction between a laser and a non-transparent thin source film deposited on a transparent plate (typically glass or quartz), which is placed in close proximity (typically about several microns) to a substrate. A process is described where a laser, such as a YAG laser, with a pulse duration of typically about 50 ns can be used to form a semiconductor junction with a depth of about 0.1 μm by using from 16 to 400 shots at a laser energy density ranging from 150 to 450 mJ/cm2. This proximity laser ablation technique can be used to deposit thin films over a large area substrate at low temperature, and one may also use a mask to block off the laser energy in areas where deposition is not desirable.
More recently, Roder et al. (Proc. of the 35th IEEE Photovoltaic Specialists Conference, pp. 3597-3599, (2010)) at the University of Stuttgart used a similar process, which they called “Laser Transferred Contacts” (LTC) or “Laser Induced Forward Transfer” (LIFT) to laser transfer a thin layer of Ni through a silicon nitride antireflection coating to form a contact to a laser-doped selective emitter region. The laser-transferred Ni contact was then electroplated with 3 μm of Ni, and then plating continued with Cu to increase the conductivity of the fingers. With this technique, a 17.4% efficient silicon solar cell was fabricated with 30 μm wide finger contacts.
Scientists at the University of Stuttgart (Hoffmann et al., Proc. of the 38th IEEE Photovoltaic Specialists Conference, pp. 1059-1062 (2012)) also demonstrated a self-doping laser transferred contact process where Sb contacts were laser transferred through a silicon nitride antireflection coating to provide a self-aligned n-type selective emitter and simultaneously formed the contacts to the front side of the solar cell. The antimony doped contacts were used as a seed layer for subsequent nickel and copper plating, and were able to produce a fine line front metallization with a finger width of 20 μm and contact resistivity as low as 30 μΩ-cm2 on a 110 Ω/sq. emitter. A green (532 nm) Nd:YAG laser was used with a pulse duration of 30 ns, as well as a green Nd:YVO4 laser with a pulse duration of 6 ns, in conjunction with an optical system that shaped the beam into a line focus in order to minimize defect creation during the recrystallization of the Si. Contact lines with widths of ˜7 μm were obtained, and Ni/Cu electroplating was used to increase the conductivity of the fingers. Solar cells with efficiencies as high as 17.5% were demonstrated.
Presently, Si solar cells with the highest efficiency are those based on combining an interdigitated all back contact structure with silicon heterojunction contacts. Panasonic recently reported obtaining a record conversion efficiency of 25.6% with such a device structure (Masuko et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.). At the same conference, Sharp reported obtaining an efficiency of 25.1% with a similar device structure (Nakamura et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.), and SunPower obtained an efficiency of 25.0% with an interdigitated back contact (IBC) silicon solar cell made using conventional diffusion processes (Smith et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.). While the processing of these high efficiency IBC solar cells were not discussed in any detail, the manufacturing costs are likely to be relatively high since the processing in each case appears to be somewhat complicated with various masking and vacuum processing steps required.
Laser processing has also been used to fabricate relatively high efficiency silicon solar cells. Benick et al. (40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.) obtained conversion efficiencies as high as 23.2% in a PERL (Passivated Emitter and Rear Locally-diffused) solar cell structure by laser doping localized rear contacts. In this process, a rear surface passivation layer consisting of a phosphorus-doped amorphous silicon carbide (a-SiCx:P) as the doping source was utilized. The front surface emitter was formed by boron diffusion, and the front side contacts were formed using photolithography and evaporating a seed layer of Ti/Pd/Ag and then plating with Ag.
Dahlinger et al. (Energy Procedia 38, 250-253 (2013)) used laser doping to fabricate an interdigitated back contact silicon solar cell with an efficiency of 22.0%. A frequency doubled (532 nm) Nd:YAG-laser with a line shaped beam (<10 μm wide) and a pulse duration of ˜50 ns was used. A thin boron precursor layer was sputtered on the rear side of the wafer, and then a laser doped p+ emitter pattern was formed. After wet chemical cleaning, a lightly phosphorus doped region was formed on both sides of the wafer using POCl3 furnace diffusion. Another laser doping process (utilizing the phosphosilicate glass grown during the POCl3 diffusion as the doping source) was used to create the n+ pattern on the rear side of the wafer. Thermal oxidation was used to drive in the diffused regions and to passivate the surface, and a plasma-enhanced chemical vapor deposited (PECVD) silicon nitride was used to form an anti-reflection coating on the front side and also an infrared reflection coating on the rear side of the wafer.
Hofmann et al. (Progress in Photovoltaics: Research and Applications; 16 509-518 (2008)) fabricated 21.7% Si solar cells used laser firing of Al through a rear surface passivation stack of amorphous silicon and PECVD silicon oxide to form localized p+ contacts. The front surface emitter was formed using phosphorus diffusion at elevated temperatures, and a thermally oxidized anti-reflection coating was used that also served as front passivation layer. The front contacts were formed by evaporating a TiPdAg finger pattern.
While it is clear that laser processing can be used to make relatively efficient silicon solar cells, the processing used to date is somewhat complicated and in all cases uses some high temperature processing along with vacuum processing. High temperature processing can create defects in the silicon wafer, which can limit solar cell performance, and vacuum processing requires relatively expensive vacuum equipment, which can limit throughput and add to the manufacturing costs.